Numerous deformation methods are known for stiffening thin material sheets and foils by profiling, among them deep-drawing, pressing and rolling with form tools such as molds and shape rolls. The drawback of these deformation technologies is that the mechanical surface pressure strongly plastifies the material sheets and foils to be profiled and degrades their surface quality.
The European patent application 0 441 618 A 1 describes a process in which polyhedral structures such as rhombic and hexagonal profiles are produced with the aid of two embossing rolls. Owing to the strong mechanical deformation, this purely mechanical forming process considerably degrades the surface quality of the materials. U.S. Pat. No. 4,576,669 suggests to feed plastic foil over a roll that carries small cups into which the plastic foil is sucked by vacuum pressure. Thus the cross-section of the foil is reduced, cracks can occur, and the inherent stability of the foil is not noticeably enhanced. Also, a device is known which serves to emboss axial beads into cans by supporting the cans on axial, rigid elements from the inside and pressurizing the outside by means of an elastic roller (DE 35 87 768 T 2). However, the inherent stability of the material furnished with axial beads in this way is insufficient because for geometric reasons beads do not yield multi-dimensional inherent stability. A process in which round, dome shaped structures are impressed in a foil by means of a perforated, cylindrical form tool and an elastic press roller also leads to a reduced cross-section of the foil and does not improve, or only very slightly improves, inherent stability because large regions remain undeformed between the round, dome shaped structures (French application no. 1,283,530).
Furthermore, a process is known in which thin material sheets or foils are profiled dent-like. In the process, the curved thin material sheet or foil is supported by line shaped supporting elements on the inside, and hydraulically pressurized from the outside. Offset quadrangular dent structures result that considerably improve the inherent stability of the material sheet (Deutsche Offenlegungsschrift 25 57 215 [Patent Application Open To Public Inspection], German printed patent specification DE 43 11 978). In principle, this dent-profiling process differs from the one described in application no. 0 441 618 A 1 in that not two mechanically acting embossing rollers are required but only a core with line shaped supporting elements on which the material sheet rests and against which it is hydraulically pressed. The curved material sheet which is supported on the inside by supporting elements arranged at intervals (rings or helix) becomes unstable due to the outside pressure. The instability results in multi-dimensional folding, and consequently offset quadrangular dent profiles form in a self-organizing process with the dent troughs evolving by themselves. As the material is only slightly stretched and upset in this process, the surface quality of the material is not degraded, or only very slightly, degraded. However, this offset quadrangular profiled material sheet (see FIG. 2 in DE 43 11 978) has no isotropic inherent stability in the plain because it is flexibly perpendicular to the continuous dent folds, and dimensionally stable parallel to these folds.
An almost isotropic inherent stability is achieved by a process in which hexagonal dent profiles are produced (published international patent application PCT/EP 94/01043, FIG. 5b and 5c). Instead of hydraulic pressure, an unprofiled, elastic cushion or an unprofiled elastomer can be used for pressurization. The supporting elements against which the material sheet is pressed are made of a flexible material which is either fixed or movable on the core. However, the hexagonal structures thus produced are not uniform.
All known profiling processes that use form tools have the essential disadvantages that the material is severely plastified, that in deep-drawing the wall thickness of the material sheet is decreased and the material can tear, that in deep-drawing bearing of the material sheet can occur, and that the surface quality of the material is degraded due to the pressure on the surface by the form tools and the severe plastification. At a given spatial depth of structure these conventional, purely mechanical, profiling methods result in a high degree of plastic deformation of the material which degrades the surface quality of the material.
Another essential drawback of the hydraulic profiling processes described in OS 25 57 215, DE 43 11 978, and PCT/EP 94/01043, is that although the degree of plastic deformation is lower than with the purely mechanical methods and the surface quality of the material is not degraded, the structures produced are not regular. The reasons for that are, among others, inevitable material non-homogeneity, wall thickness tolerances of the material to be processed, and the not absolutely even pressurization of the material sheet.